As an alternative to surfacing using TIG welding, a solution has been proposed allowing a part to be surfaced by welding which induces a restricted heat affected zone, with low levels of deformation and which furthermore can be used to surface a turbojet fan casing without it being necessary to dismantle the casing.
Such a solution essentially comprises two main steps for the surfacing, by welding, of a part made of aluminium alloy; the steps are as follows:                a layer of aluminium alloy powder is deposited manually on the part, in a surfacing zone; and        said layer of powder is welded onto said part by laser welding.        
It has in effect been observed that the laser welding carried out induced a heat affected zone and deformation which are less than those induced by T.I.G welding.
Furthermore, due the manual deposition of the powder, the equipment required to implement such a method is small in size. This allows the part that is to be surfaced to be easily accessed, even when the latter is assembled on other parts. More particularly, this allows the fan case of a turbojet to be easily accessed whilst it is still incorporated in the compressor module. Advantageously, surfacing of the fan housing is therefore carried out without the latter being dismantled from the rest of the turbojet compressor module, which allows valuable time to be saved. In addition, since there is no electric arc generated by laser welding, there is no risk of damaging the bearings.
Such a known state of the art method is recalled with reference to the FIGS. 1 to 5, whose description follows.
In these figures, a part 1, shown in section in FIG. 1, has a recess 3 in the form of a cup, where said recess has to be filled. The part 1 is, for example, a fan casing for a double-body, double-flow turbojet and belongs to the low pressure compressor module for the turbojet which comprises, in addition, the fan and the low pressure compressor (or booster).
The surfacing zone 5 of the part 1 relates to the area of the recess 3. In addition, when reference is made to the surface of the part, this is intended to designate the general surface S of this part. In the surfacing zone 5, the surface S does not therefore correspond to the bottom of the recess 3 but to the surface S of the part when it did not possess the recess 3, as shown in broken lines in FIG. 1.
A layer 8 of aluminium alloy powder 9 is deposited manually on the part 1, in the surfacing zone 5.
In practice, in order to obtain the layer 8, this can be carried out by successive depositions of at least two sub-layers 11a, 11b each formed of several beads of powder 9 which are parallel and spaced apart in steps P.
For example, in order to surface the part 1, the first sub-layer 11a formed of beads 12a spaced a distance P apart is deposited, and then the second sub-layer 11b formed of beads 12b spaced the same distance P apart is deposited, but offset by a distance P/2 in relation to the beads 12a. 
The juxtaposition of sub-layers made of beads of powder 9 means that a well densified layer 8 with a relatively constant thickness can be obtained.
Thus the height of the layer 8 of powder is calibrated relative to the surface S of the part 1. In this way the thickness of the layer 8 to be deposited can be controlled, with this thickness having an influence on the quality of the laser welding carried out. In effect, if this thickness is too small, a lack of material at the surface would result.
This calibration step naturally takes place after the step for deposition of the layer of powder and before the laser welding step. In order to carry out this calibration step, a scraper is advantageously used, comprising at least one support foot that is made to rest on the surface S of said part, and a scraper blade located further back in relation to the support foot so that the distance between this scraper blade and the surface of said part corresponds to the desired height for the layer of powder 8.
After depositing the layer 8 and having advantageously calibrated the height of the latter, the laser welding step is carried out.
For this step, for example, a diode laser 40 as can be seen in FIG. 3 is used, which offers a more regular beam/material interaction than a Y.A.G. (Yttrium Aluminium Garnet) type laser
The laser beam 41 emitted then travels over the surfacing zone 5, as shown in FIG. 4. The energy provided by the beam 41 causes the powder 9 and the adjacent portions of the part 1 to fuse and form a pool and to mix. After cooling, the desired surfacing is obtained.
According to one mode of implementation, in order to restore the aerodynamic properties of the part 1 then after the welding step the part 1 is leveled off by machining of the part in the surfacing zone 5, since the surface of the part 1 in this zone is generally not perfectly flat after welding. FIG. 4 shows the finished part obtained after the machining step.
Advantageously, as shown in FIG. 5, the use of covers, or sacrificial pieces, is envisaged which are made for example of an aluminium alloy sheet or more generally of an alloy of the same nature as the material to be surfaced. In FIG. 5, a first cover 51 and a second cover 52 are used, arranged on either side of the recess 3. The layer 8 of aluminium alloy powder is deposited in the surfacing zone 5, overflowing onto the covers 51 and 52 to create overflow zones which are not shown. The scraping of the layer 8 and the laser welding is then carried out, along the direction of propagation of the laser beam, in a manner analogous to the scraping and laser welding operations that have just been described.
In the method in FIGS. 2 to 4, the periphery of the layer of powder 8 substantially corresponds to that of the surfacing zone 5, and the interaction between the laser beam 41 and the part 1 in the transition zone between the surfacing zone 5 and the remainder of the part 1 can create micro-fissures. Such micro-fissures are a nuisance insofar as they are located at the closest edge of the surfacing zone 5, or even in the surfacing zone 5, and could weaken the latter. By making the layer 8 of powder overflow onto the covers 51 and 52, the periphery of the layer 8 is offset to outside the periphery of the surfacing zone 5. If micro-fissures are created during welding, they are located at the periphery of the overflow zones and are thus sufficiently far away from the surfacing zone 5 not to weaken the latter.
After the welding step, the covers and the powder 9 of the overflow zones which is then welded to the covers are removed. On the part 1, welded powder only remains in the surfacing zone. If necessary the part can then be leveled off by machining the part in the surfacing zone 5.
However, although this technique is particularly effective with part of small size, for example flat test pieces, the results are poorer for parts of larger size, such as casings and shells. In effect, for parts of larger size cracks are seen to appear, despite the presence of the covers, at the furthest ends of the beads; that is, at the ends which the laser beam reaches last. The presence of such cracks can be explained in particular by the fact that the pool caused by the passage of the laser rapidly heats the furthest ends of the beads, and the temperature difference between these beads and the part 1, even if located beneath the cover 52, is therefore maintained for a long period, which causes the cracks mentioned to appear. Besides, thermal pumping in large sized parts, such as casings, is greater than for parts of small size. Thermal gradients are then generated in the part, which initiates the appearance of cracks.